This invention relates generally to systems for measuring and monitoring dissolved oxygen, and more particularly, to a novel galvanic electrode system which is operated under computer control in a transient mode by programmed on-off cycling.
The need to measure and monitor concentrations of dissolved oxygen is present in a large variety of processes, including biological processes such as aerobic fermentation and waste treatment plants. However, currently available systems for measuring dissolved oxygen suffer from a variety of significant disadvantages. For example, one system in common use employs a polarographic electrode, which operates under Fick's law of diffusion through a membrane. This type of electrode requires a very stable power supply for supplying the polarizing voltage. In addition to increasing the complexity, down-time, and expense of the system, such a power supply also increases its bulk.
In operation, the polarographic electrode is energized by the application of a voltage, and the oxygen within an electrolyte in the electrode is consumed. A measured electrical current between the cathode and anode is responsive to the rate of diffusion of oxygen through the membrane of the polarographic electrode, to the surface of a cathode. It is evident that systems which use polarographic electrodes to measure dissolved oxygen are slow in reacting since it is known that diffusion through the membrane is a slow process. The slowness of operation of polarographic electrode devices is particularly disadvantageous in the monitoring of streams of fluid. In addition, such systems are not only prone to drift in calibration over time, they are also sensitive to motion of the specimen fluid, such as may result from stirring when operated in the steady state mode.
The prior art has thrust at some of these problems by employing polarographic electrode systems in pulsed mode of operation. Control over the measurement process is achieved by computer, including the duration of periods of energization and the periods therebetween. However, the algorithm used in known systems for computation of the dissolved oxygen concentration from the response of the polarographic electrode applies a linear relation constraint, which results in a long delay time, typically on the order of 1.5-3 seconds, and a long recovery time, typically greater than 3 minutes. During the delay and recovery periods, the known system is incapable of detecting dissolved oxygen. In fact, the prior art acknowledges that approximately 1.5 seconds is the shortest usable delay time if nonlinearity is to be avoided.
In addition to the foregoing, known pulsed polarographic electrode systems are incapable of reading calibration data on-line. This, when coupled with the fact that multiple programs must be run to perform the various functions of acquiring data, controlling hardware, and generating data, render the known systems difficult to operate.
A galvanic electrode is easier to use than a polarographic electrode, since it does not require application of an external voltage. Instead, the galvanic electrode generates an internal potential responsive to the oxygen flux reaching the cathode. To date, however, galvanic electrodes have been used only in a steady state mode. The steady state current depends on two primary factors, the first of which is the oxygen tension in the test solution. The steady state current is directly proportional to the oxygen concentration in the bulk solution, which dictates the driving force of the of the oxygen transport.
The second factor is the total mass transfer resistance between the test solution and the cathode. This resistance includes the resistance within the electrode, such as the electrolyte layer and the membrane, and the resistance outside of the electrode, which includes fouling and boundary layer effects. Particularly in many fermentation processes, fouling can occur at the membrane surface, resulting in an increase of the mass transfer resistance. Such fouling results in the introduction of significant error in the measurement because, in steady state operation, the outside resistances change with time and therefore have an uncalibrated effect on the oxygen flux. One known approach to reduce these effects involves the use of thicker membranes. The greater thickness of the membrane, however, disadvantageously reduces the sensitivity of the electrode, and increases the response time.
It is, therefore, an object of this invention to provide a simple and economical system for measuring accurately a concentration of dissolved oxygen.
It is another object of this invention to provide a dissolved oxygen measurement system which is readily controllable by computer.
It is also an object of this invention to provide a dissolved oxygen measurement arrangement can operate without an external voltage source.
It is a further object of this invention to provide a dissolved oxygen measurement system which can be calibrated under computer control.
It is additionally an object of this invention to provide a dissolved oxygen measurement arrangement which does not require platinum or gold in its cathode.
It is yet a further object of this invention to provide a dissolved oxygen measurement arrangement which is not sensitive to membrane fouling.
It is also another object of this invention to provide a dissolved oxygen measurement arrangement which is not sensitive to variations in speed of motion of the tested fluid.
It is yet an additional object of this invention to provide a dissolved oxygen measurement arrangement which is not sensitive to viscosity of the fluid being tested.
It is still another object of this invention to provide a dissolved oxygen measurement system which is more reliable than the presently known systems, and which is subject to less downtime.
It is a yet further object of this invention to provide a dissolved oxygen measurement system in which an algorithm is used to compute dissolved oxygen concentration.
It is also a further object of this invention to provide a system which determines a concentration of dissolved oxygen using an algorithm which is not subject to a linear relation constraint.
It is additionally another object of this invention to provide a dissolved oxygen measurement system having a low sampling delay characteristic.
A still further object of this invention is to provide a dissolved oxygen measurement arrangement having a short recovery time.
An additional object of this invention is to provide a dissolved oxygen measuring system which is capable of reading calibration data on-line.
Another object of this invention is to provide a galvanic electrode for measuring dissolved oxygen in a fluid and which is not sensitive to back diffusion of oxygen from the electrolyte.